Phosphor based lighting devices are known. For example, a light source emits light of a first wavelength(s) which is used to photo excite a luminescent phosphor layer, which then generates light of a different (desired) wavelength(s).
In such devices, a solid state pump source (e.g. a laser diode) can be used as a source of monochromatic light for the system. This source light is used to pump a conversion phosphor that absorbs the pump light and re-emits light at one or more desired wavelengths. The light emitted by the phosphor typically has improved luminous efficacy compared to that emitted by the pump source. By varying the monochromatic source and the phosphor, a range of emitted wavelengths can be achieved as an output of such a device.
Semiconductors typically used in the pump source include GaAs, InP, GaSb and GaN. Commonly used phosphors are garnet pellets such as Yttrium Aluminium Garnet (“YAG”), but also quantum dots such as CdSe/CdS or InP and organic dyes are employed.
Phosphors are found in applications such as white light emitting diodes (LEDs) and full down-conversion based devices. The latter is considered and proved to be very promising for the production of high-color-purity light, especially in wavelength ranges in which direct radiation from non-converted LEDs is relatively inefficient, i.e. in the so-called “yellow gap”. Typically, amber LEDs are used in automobile applications or in traffic signals and are believed to be promising for illumination of photolithography rooms.
By way of example, Ce3+ doped yttrium aluminum garnet, Y3Al5O12 (YAG:Ce), is an important phosphor for lighting applications because of its superb luminescent properties, chemical durability, and thermal stability. It has an absorption peak around 460 nm while its emission spectrum spans the range 500-750 nm.
It is generally desirable to maximize the absorption of light by the phosphor in order to generate as much converted light as possible. One direct way of enhancing the absorption is to increase the amount of Ce3+ in the YAG crystal. However if the concentration is higher than a threshold (typically >5 mol %), the YAG:Ce starts to exhibit concentration quenching, resulting in a reduced quantum efficiency and decreased emission intensity. For a fixed percentage of doping, total absorption is prevented by the amount of reflected and transmitted pump light which escapes the phosphor layer.
To limit the amount of light escaping after transmission through a YAG:Ce pellet it is in general convenient to pump the phosphor at the wavelength of its absorption peak and use a long pass filter in order to reflect and recycle the unabsorbed pump light. However this introduces a natural loss, known as quantum deficit, in the efficiency of the device, which is associated with the Stokes shift characteristic of the phosphor. The quantum deficit is defined as the difference between the excitation wavelength and the maximum emission wavelength. The Stokes' shift characteristic of the phosphor represents the difference in wavelength (or energy difference) between absorption and emission at the phosphor (based on the wavelength at which the imaginary component of the refractive index of the medium is maximum).
This quantum deficit is generally considered unavoidable. The quantum deficit is important because it appears in the formula for the power efficiency of the phosphor-converted LED, where it is quantified through the ratio of the excitation wavelength to the centroid of the emission spectrum.
In prior art lighting devices the intensity of the photo luminescence is dictated only by the intensity of the incident light from the pump light source. In order to achieve efficient absorption of the pump light, the thickness of the phosphor needs to be comparable to its absorption length.
For commonly used phosphors, like yttrium aluminum garnet, the thickness of material required for sufficient absorption of the pump source is in the order of several tens of microns. To minimize this thickness, the phosphor is pumped at the wavelength at which its absorption coefficient is at a maximum and the quantum deficit is also large. The quantum deficit represents a limit to the efficiency of such a device and causes the heating of the phosphor. Upon overheating (the temperature of the phosphor may exceed 300° C.), the efficiency of the phosphor drops significantly and results in additional power loss and uncontrolled further heating which may damage the lighting device.
US2004/0150997A1 discloses a LED with a phosphor layer that receives excitation light. The phosphor layer emits visible light when illuminated with the excitation light. A polymeric multilayer optical film enhances the efficiency of the LED by reflecting light that would otherwise be wasted back onto the phosphor layer.